The human T-cell lymphotropic virus type 1 (HTLV-I) is epidemiologically associated with an aggressive and fatal T-cell type leukemia/ lymphoma designated Adult T-cell Leukemia/ Lymphoma (ATLL). HTLV-I infection is also associated with a progressive myelopathy designated Tropical Spastic Paraparesis/ HTLV-I Associated Myelopathy (TSP/HAM). It is estimated that 20 to 30 million people worldwide are infected with HTLV-I. HTLV-I possesses unusual features that would not predict its survival in an immune-competent host: The virus is poorly infectious but elicits a vigorous humoral and cellular host immune response. In addition, HTLV-I is mainly replicated in vivo through division of infected cells and therefore presents a very low antigenic variability. However, in spite of these apparent disadvantages the virus has persisted in humans for more than 100,000 years, indicating that HTLV-I has efficiently adapted to its host. We have recently found that HTLV-I has evolved a protein, p30, that interacts specifically with the tax/rex viral RNA encoding positive regulators of virus expression. Because p30 is unable to shuttle out of the nucleus, tax/rex RNA is trapped in the nucleus and expression of these proteins is inhibited. The goals of this study are to uncover the molecular mechanisms involved in p30-viral RNA interactions and p30 effects on transcription leading to inhibition of virus replication and latency. Four specific aims are proposed. Aim 1) Investigate biochemical characteristics of p30-RNA interactions. Define the minimal p30-response RNA sequence within HTLV-I provirus and identify the domain(s) of the p30 protein involved in these interactions. Aim 2) Analyze transcriptional and post transcriptional effects of p30 on HTLV-I replication in vivo and in vitro. Specific mutants unable to interact with transcriptional regulator CBP/p300 or with viral RNA will be generated in order to discriminate each activity of p30. Aim 3) Investigate molecular determinants for the nuclear/nucleolar retention of p30 and the effects of p30 on Rex-RNA binding activity or Rex functions and vice versa. Aim 4) Study the kinetics of p30 and tax/rex RNA expression in relation to virus expression in vitro using molecular clones mutated for p30. Kinetics of viral RNA expression will be studied in vivo in different lymphoid tissues collected longitudinally from an infected squirrel monkey model and in asymptomatic or HTLV-I-infected patients with neurological or hematological disorders.